Republicans have opened their convention to nominate Donald Trump for US president as dissident delegates pursued one last chance to deny the front-runner.

A day after a deadly ambush of police in Louisiana, Republican National Committee chairman Reince Priebus welcomed delegates to the convention hall with a brief acknowledgement of the "troubling times" swirling outside. The chairman called for a moment of silence out of respect for "genuine heroes" in law enforcement.

"Our nation grieves when we see these awful killings," he said.

Weeks of racial tensions and violence are shadowing the Republicans' long-awaited showcase of their presidential pick and putting both participants and the convention city on alert.

True to form, Mr Trump himself provided the first curveball of the week, announcing he will make an unexpected swing to the convention hall on Monday night to introduce his wife, Melania, on the first night of speeches.

"I want to watch," Mr Trump said on Fox News. "It is going to be very exciting."

This week will belong to Mr Trump - his chance to stand at the pinnacle of American politics in a triumph that few could have imagined when the New York billionaire entered the race a year ago. He will take on Democrat Hillary Clinton in November.

The line-up of speakers is aimed at showing off the man behind the mogul, his advisers say. Several family members and friends are slated to speak to his character and reveal a side of Mr Trump that Americans may not know.

Along with Melania Trump, Monday's opening night speeches include a mix of figures linked broadly under the theme of "Make America Safe Again." They include Senators Joni Ernst of Iowa and Jeff Sessions of Alabama, as well as immigration advocates, a Marine who fought in the Benghazi attack and entertainers, including actor Scott Baio and Willie Robertson, star of Duck Dynasty.

However, the speakers highlight the wedge Mr Trump's candidacy has driven through the Republican party.

Many Republican leaders, party elders and rising stars have steered clear of Cleveland. Others have come to meet with donors and rank-and-file but have kept heir distance from the main event.

New Mexico Governor Susana Martinez came to Cleveland but had no role on the programme.

When House Speaker Paul Ryan spoke to Wisconsin delegates on Monday morning, he made no mention of Mr Trump in his remarks.

Ohio Governor John Kasich, a vanquished Trump rival, planned several public appearances but was not going to step inside the Quicken Loans Arena.

Top Trump adviser Paul Manafort called Mr Kasich's behaviour "petulant" and he brushed off the absence of former presidents George HW Bush and George W Bush.

"Certainly the Bush family, we would have liked to have had them. They're part of the past. We're dealing with the future."

"When we leave here, by and large, it's going to be a united Republican Party," he said.

That remained to be seen. Disaffected delegates continued to try to derail Mr Trump.

All delegates are due to vote on Monday afternoon on the rules that will govern the convention week, and insurgent delegates are circulating a petition to force a state-by-state vote - a threat to disrupt floor proceedings even if it fails.

Some rebellious delegates are threatening to walk out.

"We won't sit around and coronate a king," said Colorado delegate Kendal Unruh, who like many insurgents has backed vanquished contender Senator Ted Cruz of Texas.

The roll call vote on the nomination is expected on Tuesday, with Mr Trump scheduled to close the convention with an acceptance speech late on Thursday.

Mr Trump is gaining the nomination during a summer of unsettling violence at home and abroad. In a matter of weeks, Americans have seen deadly police shootings, a shocking ambush of police in Texas and escalating racial tensions, not to mention a failed coup in Turkey and the Bastille Day attack in Nice, France. The killing of three police officers in Baton Rouge, Louisiana, early on Sunday added to the sense of a nation on edge.

Mr Trump has sought to capitalise on the mood by casting himself as the "law and order candidate" and blasting Democrats for weak leadership in a crisis.

On Monday, he quickly linked the shooter to "radical Islam," despite early indications the man had no known ties to any extremist group.

Former Marine Gavin Eugene Long "seems to be a member of that group also. It seems to be something going on there," Mr Trump said on Fox News.

Campaigning in Cincinnati on Monday afternoon, Democrat Mrs Clinton called for an end to the "madness," saying that if elected she would use all her powers to hold those who kill police officers legally accountable.

"They represent the rule of law itself. If you take aim at that and at them, you take aim at all of us," Mrs Clinton told civil rights activists at the annual convention of the NAACP. "There can be no justification, no looking the other way."

Mrs Clinton's campaign is spending about one million US dollars (754,000) on TV ads in Ohio this week, according to Kantar Media's campaign advertising tracker.

Several hundred Donald Trump supporters gathered on Monday for the first major pro-Trump rally in the convention city.

A few openly carried guns as allowed under Ohio law. The president of the police union had asked Mr Kasich to suspend the law allowing gun owners to carry firearms in plain sight. But Mr Kasich said he does not have that authority.